The invention relates a method for adjusting an actuator which can move between two end positions, which is displaced into one end position and which can be moved to the other end position by means of an adjusting unit.
Actuators of the type in question here, which are displaced into one end position and can be moved into the other end position by means of an adjusting unit, must therefore be held in a desired position by actively activating the adjusting unit. From a held position, it is possible either to bring about adjustment into the one end position by suspending the activation of the adjusting unit, or to bring about adjustment into the other end position by increased activation of the adjusting unit. A convenient way of activating such an adjusting unit, which may, for example, operate electromagnetically, is actuation with a pulse-width-modulated signal. Depending on the pulse duty factor of the pulse-width-modulation, adjustment is carried out into one end position or the other end position. If the actuator is to be held in one position, the adjusting unit must be actuated with a retaining pulse duty factor.
The actuators which are described are preferably used in devices for camshaft phase adjustment in internal combustion engines. Such a camshaft phase adjuster is described, for example, in DE 43 40 614 C2. It is a typical example of an actuator which is influenced by an adjusting unit and in which dead times and delayed response require limitation of the maximum achievable adjustment speed and consequently corresponding parametrization of the associated adjuster.
Owing to these dead, times and the delayed response, it is not possible to equip the adjuster which adjusts the actual position of the actuator with an integral component as otherwise an unstable system would be produced. Instead, a certain maximum control error, below which the adjuster is not active, is permitted.
However, this procedure leads in such cases to difficulties in which the actual position of the actuator cannot be measured continuously but rather only sampling is possible. There are then cases in which, despite repeated adjusting intervention, the desired position is not reached but there is instead a quasi-steady or drifting state of the actuator in which the actuator exhibits a constant control error or a continuous movement to an end position.
Accordingly, there is a need, for a method of adjusting an actuator of the type described, with which precise adjustment to a desired position can be reached without quasi-steady or drifting states occurring.
Other needs will become apparent upon a further reading of the following detailed description taken in conjunction with the drawings.
The invention is based on the idea that the retaining pulse duty factor is, of course, the same for all the operating states of the actuator only in the rarest of cases. Although an actuator can be configured in such a way that the retaining pulse duty factor is the same for all the actual positions of the actuator, this cannot be achieved for all operating conditions, for example temperatures, supply voltages, hydraulic pressures or the like. If the retaining pulse duty factor does not have precisely the value which is necessary to keep the actuator in an actual position, it will move toward an end position. A faulty retaining pulse duty factor is thus the cause of a quasi-steady or drifting state. A quasi-steady state is found if, despite repeated adjusting intervention, a minimum control error is continuously exceeded. The retaining pulse duty factor is then changed until the control error drops below a threshold value.
In the case of the drifting state of the actuator, the drift behavior is determined and the retaining pulse duty factor is correspondingly corrected until the desired position is maintained precisely within a desired framework.
The difference between a quasi-steady and drifting state is caused by the fault in the retaining pulse duty factor. When there is a relatively large fault in the retaining pulse duty factor, a quasi-steady state will be established. Between the times of the sampling measurement of the actual position, the actuator drifts out of the acceptable control error so quickly that a constant control error is measured despite repeated control interventions. On the other hand, in the drifting state, the fault of the retaining pulse duty factor becomes relatively small. Here, the movement of the actuator out of the desired position takes place so slowly that one or more measurements exhibit an actual position within the acceptable control error. This makes it possible to determine the drift behavior, and calculate precisely the necessary correction of the retaining pulse duty factor from it.
As the retaining pulse duty factor may need to be corrected not only as a result of operating states of the actuator, but it may also need to be changed due to a defect in the actuator, a defect in the actuator is detected if the change in the retaining pulse duty factor appears necessary beyond a specific pulse width modulation. The actuator is also defective if correction of the retaining pulse duty factor is repeatedly necessary over a time period, that is to say no fixed retaining pulse duty factor can be found during the control over a relatively long time period during which the acceptable control error is maintained.
Owing to the dead times and the delayed response behavior of the actuator, it would of course be desirable to configure the adjuster to be as immune to oscillation as possible. On the other hand, in many applications, for example in the aforementioned camshaft phase adjusters, rapid re-adjustment into a new desired position is required. These, in themselves, contradictory objectives can be achieved in one preferred development by virtue of the fact that large jumps in the desired position can be achieved by pilot control and the adjuster is active only in a narrow range around the respective desired position. Here, the adjuster can be permitted only a certain maximum change of the pulse width modulation, which has positive effects on the stability. This maximum change is preferably dependent on the adjustment to be brought about in the actual position, which leads to the actual position being adjusted in a non-oscillating way to the, desired position, even when there are relatively long dead times.
In an actuator which is embodied as a camshaft phase adjuster, the sampling of the position of the camshaft, and thus the determination of the position of the actuator, generally takes place once or twice per revolution of the camshafts, in that a semicircular disk which is attached to the camshaft is sensed. The selection of the retaining pulse duty factor can be given a two-stage configuration for such a camshaft phase adjuster. On the one hand, a basic value for the retaining pulse duty factor is obtained from a basic characteristic diagram which takes into account operating parameters of the internal combustion engine, for example operating temperature, oil pressure, battery voltage or the like. On the other hand, the aforementioned correction of the retaining pulse duty factor can be obtained from an adaptation characteristic diagram which covers the constant control error or one or more parameters which characterize the drift behavior. Advantageous refinements of the invention are the subject matter of the subclaims.